The hunting of the snArc

Volcanic and intrusive rocks with geochemical signatures typical of modern continental or oceanic arcs are uncommon in the Archaean and the archetypal Archaean granite-greenstone dome-and-keel architecture has no modern analogue. Proposed Archaean ophiolites, Atlantic-style passive margins, overprinting thrust and fold belts, blueschists, ultra high-pressure rocks, paired metamorphic belts, orogenic andesites, and subduction-zone mélanges that typify Phanerozoic plate margins are rare to absent. Since there is little evidence for Archaean subduction zones, we consider Archaean arcs to be ‘Snarks’, imaginary constructs with no objective existence. Some Archaean folds and upper-mantle structures interpreted as evidence for regional thrusting may equally well have developed during horizontal extension of low-viscosity crust. Nonetheless, there are clearly many Archaean terrains exhibiting fabrics formed by bulk shortening and some cratons contain terranes with contrasting histories. Given the absence of evidence for Archaean subduction, what could be a plausible driving force for compression and terrane accretion? Cratonic mobilism in response to mantle convection currents offers a possible solution to this paradox. Once a proto-craton develops a deep high-viscosity mantle keel, it would become subject to pressure from mantle currents and could drift. Immature cratons or oceanic plateaux would not have a strong mantle keel and so would be static. So, contrary to conventional wisdom, we consider that Archaean cratons are not immobile nucleii along whose margins ‘mobile belts’ form by subduction-zone accretion. Instead, we propose that Archaean cratons were the active tectonic agents, accreting and subcreting basaltic plateaux, other proto-cratons, and heterogeneous mantle domains as they drifted. In this model, accreted terranes and structures indicating bulk shortening would be concentrated at the cratonic leading edge, with oblique and strike-slip shear zones at the sides, extension and possible seafloor-spreading in the lee, and major oblique-slip shear zones in the interior developed as a result of the imposed stress field. Overridden oceanic plateaux would be thrust (subcreted) deep enough to melt in the garnet field and generate syntectonic pulses of tonalite–trondhjemite–granodiorite (TTG), contributing to craton growth and stabilization.

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► Archaean cratons drift due to pressure of mantle flow on their keels.
► Oceanic plateaux accrete to mobile Archaean craton leading edges.
► Archaean arcs are snarks: often sought but never found.
► The paradox of subductionless Archaean terrane accretion and shortening is eliminated.